Ceramic adhesives have properties such as good mechanical strength and high temperature stability, making them suitable for high technology applications. However, higher porosity and lower fracture toughness than desirable are some of the limitations of ceramic adhesives. Hence, recent work in this field has been directed to the development of tougher and more reliable ceramic adhesive materials.
In the past, ceramic adhesives have been used in various high temperature applications. However, most of these applications have been of a nature where the ceramic adhesive has been subjected mainly to thermal stresses, and not a combination of thermal and mechanical stresses, chemically corrosive environment and abrasive wear. U.S. Pat. No. 5,006,423 issued Apr. 9, 1991 to Draskovich, discloses a refractory cement formed from a mixture of sodium silicate in water and a ceramic powder, such as silicon nitride, silicon carbide or silicon dioxide. Draskovich provides this refractory cement for bonding metallic instrumentation to ceramic components of a gas turbine engine, for use at temperatures below about 2500 .degree. F. Draskovich does not solve the problem of bonding metallic instrumentation to metallic parts, where the ceramic adhesive must be durable and long-lasting, in spite of the differences in thermal coefficient of expansion and thermal conductivity, between the metallic instrumentation, the ceramic adhesive and the metallic component.
It has been known that ceramic composites can be reinforced by using fibers. It is also known that silicon carbide fibers toughen the material through the mechanisms of crack deflection and fiber pullout. However, silicon carbide fibers tend to soften at temperatures of about 850 .degree. C. and lose their toughening effect. Several inventors have discovered the advantages of using silicon carbide whiskers (SiC whiskers) in ceramic compositions. U.S. Pat. No. 5,108,963 issued Apr. 28, 1992 to Fu et. al, discloses a SiC whiskers reinforced alumina based ceramic composite which shows improved mechanical properties and sinterability through the addition of chromia. However, this ceramic does not have any adhesive properties. Further, it requires a sintering temperature of about 1350.degree. C. Such a high temperature destroys the sensor mounted in the engine part.
U.S. Pat. No. 4,231,800 issued Nov. 4, 1980 to Holt et. al, discloses a dry heat setting refractory for securing nozzles to nozzle blocks in steel pour ladles. Holt discloses a refractory composition comprising magnesite and hydrated sodium silicate in a weight ratio in the range of 94:6 to 98:2. Holt preferably uses hydrated sodium silicate so that the refractory material is dry and a bond is formed when hot steel engages the dry refractory. Although the type of ceramic adhesive disclosed by Holt may be suitable for bonding nozzles to nozzle blocks in steel pour ladles, it has been found to be not useful in mounting and bonding instrumentation on components inside an internal combustion engine. For example, an adhesive used for mounting a motion sensor in a piston-ring groove, is subjected to temperatures in the range of about 500.degree.-600 .degree. C., combustion gases and lubricant flow. All of these work together to create a tribological wear environment that eventually erodes the adhesive and destroys the bond between the instrumentation and the engine component.
In the present invention, it has been found that a ceramic adhesive formed from a mixture of an alkali silicate in water and magnesium oxide, can be substantially toughened by the addition of silicon carbide (SiC) whiskers. Further, because this ceramic adhesive is cured at low temperatures, it is believed that there is no chemical bonding of the whiskers with the ceramic matrix. Hence, the desirable properties imparted by the SiC whiskers, like impeding crack propagation through mechanisms such as crack bridging, crack deflection and whisker pullout, are exploited to impart a combination of good mechanical, thermal, chemical and abrasion resistance properties to the ceramic adhesive.
It is also known to bond sensors to components by means of other adhesives, such as organic epoxy glues. Epoxy glues have good bonding characteristics, but they are limited to temperatures below about 260 .degree. C. It has been also discovered that an epoxy can be used to seal the porosity of a ceramic adhesive, resulting in an overall improvement in the mechanical, thermal, chemical and abrasion resistance properties of the sealed ceramic adhesive.
It is desirable to have a tough ceramic adhesive for bonding instrumentation such as ring motion sensors, pressure sensors, thermocouples, and liner gap sensors, on engine components such as cylinder walls and pistons, that can withstand thermal and mechanical stresses, and the deleterious effect of combustion gases, engine lubricants and abrasive wear. The present invention is directed to overcome one or more of the problems as set forth above.